The invention relates to a projective or extendable arrangement in a radio communication device such as a hand-portable telephone. The extendable arrangement forms part of an antenna system for the radio communication device. Such an extendable arrangement includes a movable and, preferably, non-conductive support means and conductive means supported by said support means. In particular, the support means may be a foldable or slidable cover portion of a hand-portable telephone.
There are general problems of providing, in modern mobile radio communication devices, such as hand-portable mobile telephones, an efficient antenna system that fulfills the demands of compactness, ease of portability and use, radiation distribution or directivity, operability in stand-by and talk (communication) modes, operability in plural communication systems, antenna spatial diversity, etc. The demands will vary depending on a selected radio communication system environment wherein the device is intended for operation. For instance, predetermined frequency bands, bandwidths, and system architecture or infrastructure set constraints and requirements to the performance of the antenna system.
In general, a mobile telephone includes a main housing holding an antenna as well as control and radio frequency circuitry. The main housing may also incorporate communication input means, e.g., a microphone or a key pad and communication output means, e.g., a loudspeaker or a graphics display. Of course, considering the many purposes one may include in a mobile radio communication device, the input and output means may handle various kinds of input and output quantities. In case the radio communication device is to be hand-held or located close to a part of the users body, there are problems of screening the radiation to and from the device and uncontrollable variations in the operation environment of the device affecting such parameters as, for instance, resonant frequency and impedance of antenna elements. It is also an unwanted effect that the radiation of the antenna system is absorbed in the body of the user. Problems of similar type may also occur if the radio communication device is operated directly from a communication terminal such as a stationary or portable computer.
There are further problems in designing an efficient antenna system to be mounted on or inside a radio communication device, especially when the device is relatively short in relation to a wavelength of a frequency band at which it is intended to operate. In fact, two common cellular telephone systems operate at approximately 800 and 900 Mhz, which gives wavelength of approximately 38 cm and 33 cm, respectively. It is common to aim at arranging the antenna system to include a dipole type antenna having a longitudinal extension of approximately half a wavelength. Frequently, part of the telephone housing or at least a conductive interior thereof constitutes a ground plane means of the antenna system. Longer antenna systems may also be advantageous. Physically shorter systems are achievable if at least some of the antenna elements are, e.g. folded or helical or meander-shaped, maintaining a desired electrical length.
If the radio communication device is to be hand-held, a suitable longitudinal extension of the device to be held is approximately equal to the distance across the hand, i.e., in the order of 10 cm. If the device should also perform the function of a telephone receiver, it should extend during use approximately between the ear and mouth of the user, i.e., optimally it should be slightly longer 10 cm.
These and other constraints and desires form the demands on a mobile radio communication device such as a mobile telephone. There have been many attempts in the prior art to provide efficient antennas meeting the demands. The result will always be a compromise optimized to meet a selected specification since a constant and ideal antenna environment is not achievable.
Below there will be briefly described two prior art antenna systems, which form part of the basis for the improvements obtained through the present invention. Even further prior art devices are indicated by references.
A first prior art antenna system for a portable cellular telephone is disclosed in U.S. Pat. No. 4,868,576 and includes a quarter-wavelength ground radiator and a helical coil capacitively coupled to an extendable half-wavelength radiator. The extendable radiator includes a tightly wound coil having a small diameter. The ground radiator includes a meander-shaped conductor extending perpendicularly from a feed point of the helical coil. The ground radiator and the helical radiator are connected via a feed transmission line to duplexer circuitry in the device. The antenna system disclosed therein is intended to improve the radio function of a small size portable radio and provide immunity to hand induced radiation losses.
A second prior art antenna system for a portable cellular telephone is disclosed in U.S. Pat. No. 5,554,996. That telephone includes a main housing and foldable flap to extend out from the main housing during a telephone call such that one end of the flap is hingedly connected to the main housing and the other end is to be located near the users mouth in talk position. For that reason the flap may advantageously include voice pickup means. The antenna system includes a first antenna having fixed and slidable parts at one end of the main housing and a second antenna at the other end of the housing where the flap is connected. The second antenna has two portions: one feed portion in the main housing and one parasitic radiator portion in the flap. The first and second antennas are intended to provide antenna diversity to the telephone. This means that the first and second antennas operate independently of each other and circuitry in the telephone selects a signal from the antenna providing the best signal at a given instant.
In the prior art there are several other hand-portable radio telephones which include a flip part covering the keypad when the telephone is not operated and for being moved away from the keypad when a user wishes to operate the keypad or answer an incoming call. Such arrangements are disclosed in patents and published patent applications such as U.S. Pat. No. 5,561,436 (Phillips), U.S. Pat. No. 5,337,061 (Pye), U.S. Pat. No. 5,561,437 (Phillips et al.), U.S. Pat. No. 5,014,346 (Phillips et al.), U.S. Pat. No. 5,649,306 (Vannatta et al.), U.S. Pat. No. 5,572,223 (Phillips et al.), U.S. Pat. No. 5,542,106 (Krenz et al.), U.S. Pat. No. 5,508,709 (Krenz et al.), WO 97/23016 (Geotech Communication, Inc.), and WO97/26713 (Ericsson, Inc.). There is also known from commercially available hand-portable radio telephones a slide part acting as a movable cover similarly to the flip parts referred to above.
Some of the telephones including flaps have a dual antenna diversity function, wherein one antenna is located in an upper portion of the telephone body and one in a flip part hinged at a lower portion of the telephone. Others have a main antenna element in or very closely coupled to the flip part fastened at a lower portion of the telephone. In the disclosed or known arrangements the antenna is coupled to circuitry (hot wire) of the telephone and part of the telephone main body is acting as a ground plane or ground means. It is a general problem to get a satisfactory antenna performance with hot wire antenna elements integrated in the flip part when folded together. This is increasingly difficult as dual and multiband operability, e.g. GSM in combination with PCN, is to be achieved.
However, in spite of all effort in the prior art towards providing diversity or main antenna elements in movable keypad covers, no disclosures or known devices account for the possibility or any means of arranging an improved ground means in the movable cover.
It is thus a main object of the invention to provide improvement in antenna performance in an antenna system having at least one extendable portion. The inventive improvement is particularly efficient in a device having a main housing including a ground portion, a fed antenna element at one end and an extendable portion at the other end. A particular object is to reduce adverse effects of currents being induced on a main body of a radio communication device in use. Another particular object is to provide a device that minimizes SAR. Yet another object is to provide operability in more than one frequency band.
These and further objects are attained by a system or device of the appended claims. The inventive extendability of the ground portion of a radio communication device enhances antenna performance. The invention provides the advantages of improving antenna performance parameters such as VSWR, antenna gain, limitation of SAR (Specific Absorption Rate), and immunity to screening as well as hand induced or other radiation losses. The dependent claims recite various enhancements of the invention in attaining above-mentioned objects. Several different types of ground structures may be employed alternatively in the invention, as will be evident from the detailed description below.
The extendable ground means is preferably coupled capacitively to ground means at a location having an elevated voltage. Thus, it should be noted that a voltage potential locally of a ground conductor in a radio device is not necessarily constant and equal to 0 volts in an RF signal sense. Rather, ground means in the context of this disclosure is a conductive portion coupled primarily to signal ground of radio transceiver circuitry. Generally, antenna feed circuitry is connected to the antenna, at an antenna feed point, by a hot connection and a ground or common connection. If required an impedance matching interface is arranged between the antenna and the feed circuitry.
There may advantageously be a balun means interconnecting to hot and ground elements in the inventive system.
In this disclosure it is to be understood that the antenna system of the invention is operable to transmit and/or receive radio signals. Even if a term is used herein that suggests one specific signal direction it is to be appreciated that such the situation covers that signal direction and/or, if applicable, its reverse.